Lebanon's education minister Elias Bousaab became the latest in a series of politicians to say that Islamic State (Isis) militants may be infiltrating migrant boats carrying would-be refugees to Europe. His and similar claims have been used by nationalist parties across the EU to advocate for tougher immigration policies.

In Britain, Ukip has warned more vocally than any other party about the risks of Islamist intrusions. Launching the party campaign for Britain's EU referendum at the beginning of September, leader Nigel Farage said there was a "very genuine fear" that IS could exploit the migrant crisis to carry out atrocities.

"When [IS] say they will use the migrant tide to flood Europe with 500,000 of their own jihadists, I think we better listen," he said, citing threats made by IS supporters online. "Five hundred thousand may not be realistic but what if it's 5,000, what if it's 500?" he asked.

"There have been no confirmed cases, zero!"

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More than 500,000 migrants have crossed EU borders this year, adding to the 280,000 who arrived in 2014, according to the 28-nation bloc border agency. Many fled war in Syria and Iraq ? where IS has a strong presence ? or boarded rickety boats for Italy in Libya, where the extremist group is also active.

The possibility that even a tiny number of them are trained jihadists is indeed a matter of concern for European governments. However there is little proof this is actually happening, according to officials and analysts.

Italy, which with Greece is the main entry point for asylum seekers travelling via sea, hasn't had any case of known terrorists disembarking from a migrant boat over the last two years, a lawmaker with inside knowledge of related investigations told reporters.

"There have been no confirmed cases, zero!" said Giorgio Brandolin, the deputy chairman of a cross-chamber parliamentary committee on immigration and security. "Terror groups spend money on training militants; it makes no sense for them to send them over on death boats, risking them drowning on the way," he said.